The NAS protocol used for UE/MME interaction has a security protection mechanism comprising integrity protection and/or ciphering of messages. In normal ciphered operation all messages shall be ciphered as soon as a security context has been activated for a specific NAS connection.
It is also specified in 3GPP TS 24.301 that ciphering is an operator option. Thus SAE needs to support un-ciphered operation in the NAS protocol.
Different methods to solve un-ciphered operation may be selected and one way is to always use non security protected NAS messages. There are two general types of NAS messages, plain NAS messages that have no security protection and security protected NAS messages that have integrity protection and/or ciphering applied.
If there is no common way to solve un-ciphered operation mode in SAE there will be interoperability problems between UE and MME and between different vendors, effectively making un-ciphered operation impossible.
If a method is chosen where non security protected NAS messages are used for all messages in un-ciphered operation mode, it implies that the MME implementations will be more complicated and needs to be able to distinguish between ciphered and un-ciphered operation for most NAS messages.
Furthermore, if using plain NAS messages in un-ciphered operation, integrity protection of the NAS messages will not be available and exceptions to those rules would also have to be implemented.